LCD units are roughly categorized into two groups including a transmissive LCD unit and a reflective LCD unit. In general, the transmissive LCD unit includes a backlight source, and controls the intensity of backlight passed by a liquid crystal (LC) layer to display an image thereon. The reflective LCD includes a reflector that reflects light incident from outside the LCD unit, and uses the light reflected by the reflector to display an image thereon. The reflective LCD unit, which does not use the backlight source, has the advantages of lower power dissipation, smaller thickness and smaller weight. However, the reflective LCD unit suffers from a lower visibility in a dark environment due to use of the dim ambient light.
A transflective LCD unit is known as a LCD unit having the advantages of both the transmissive LCD unit and reflective LCD unit (for example, refer to JP-2003-344837A (Patent Publication 1)). The transflective LCD includes a transmissive area and a reflective area in each of the pixels in the LCD unit. The transmissive area passes the light emitted by the backlight source to use the backlight from the backlight source as the light for display. The reflective area includes a reflector that reflects light from outside the LCD unit, to use the light reflected by the reflector as the light of display. In the transflective LCD, the backlight source is turned OFF in a bright environment to save the power source, and turned ON in a dark environment to use the backlight source for display of an image in the dark environment.
Modes for driving a LCD unit include an IPS mode such as a lateral-electric-filed mode and a fringe-electric-field mode. The IPS-mode LCD unit includes, in each pixel, a pixel electrode and a common electrode which were juxtaposed on the same substrate and apply therebetween a lateral electric field to the LC layer. The IPS-mode LCD unit achieves a wider-viewing-angle characteristic compared to a twisted-nematic-mode (TN-mode) LCD unit, due to a lateral rotation of LC molecules in the LC layer, i.e., rotation in the direction parallel to the substrate surface.
There is a known technique for driving a transflective lateral-electric-field-mode LCD unit including the transmissive area and reflective area, which may be referred to as simply transflective LCD unit hereinafter, in a normally-black drive mode, as described in JP-2006-171376A (Patent Publication 2). In this technique, the normally-black drive mode is achieved by allowing the LC layer in the reflective area to act as a λ/4 film with respect to a light having a wavelength (λ) of 550 nm, allowing the LC layer in the transmissive area to act as a λ/2 film with respect to a light having the same wavelength, and interposing a λ/2 retardation film between the polarizing film and the LC layer in the reflective area.
It is also known to drive the transmissive area and reflective area in the transflective LCD unit by using an inverted driving scheme without using the λ/2 film (refer to JP-2007-041572A (Patent Publication 3)). In this technique, the reflective area and transmissive area in each pixel are provided with respective data lines for supply of data signals, respective switches for switching between the data lines, respective pixel electrodes, and respective common electrodes. The LC layer in the reflective area and the LC layer in the transmissive area are driven in an inverted drive scheme wherein the intensity of the electric field applied to the LC layer by the pixel electrode and the common electrode is opposite between the reflective area and the transmissive area. The inverted driving scheme is such that when the LC layer in the reflective area is not applied with a voltage to display a bright state (white), the LC layer in the transmissive area is applied with a full voltage to display a bright state (W), and vice versa, and such that when the applied voltage in the reflective area is lowered from the full voltage, the applied voltage in the transmissive area is raised from the zero volt, and vice versa. The inverted driving scheme achieves the bright state in both the reflective area and transmissive area as well as the dark state in both the areas.